the "Unwanted" folder in my email

How can I stop emails from going to this?? For some reason after a few back and forth emails with some people their emails started going to this unwanted folder..... I'm missing very important emails and people are getting mad!! why on earth are they fine at first and going to my inbox and then all the sudden they're going to the unwanted folder?? I NEVER marked them as spam... this is irritating! How do I get these peoples emails to stop going there!